Each is a simplified version of for-each.
Calls a function for each element in the argument collection.
When the "each" ends, f.ret is pointing back to the argument
collection.
set l []
each [ 0 1 2 3 4 5 6 7 ]
pushr l (2 * elt) if elt % 2 == 0
the var l will yield [ 0, 4, 8, 12 ] after the each
the field ret will yield [ 0, 1, 2, 3, 4, 5, 6, 7 ].
set l []
each { a: 'A', b: 'B', c: 'C' }
pushr l (+ key val idx)
the var l will yield [ 'aA0', 'bB1', 'cC2' ] after the each
Iterating blocks are given 3 to 4 local variables.
A block iterating over an array will receive elt (the current element
of the iteration), idx (the zero-based index of the current element),
and len (the length of the array).
A block iterating over an object will receive key (the current string
key), val (the current value), idx (the zero-based index of the
current key/val), and len (the length of the object).